IEEE 802.11 is a set of media access control (MAC) and physical layer (PHY) specification for implementing wireless local area network (WLAN) communication, called WiFi, in the unlicensed (2.4, 3.6, 5, and 60 GHz) frequency bands. The standards and amendments provide the basis for wireless network products using the WiFi frequency bands. For example, IEEE 802.11ac is a wireless networking standard in the 802.11 family providing high-throughput WLANs on the 5 GHz band. Significant wider channel bandwidths (20 MHz, 40 MHz, 80 MHz, and 160 MHz) were proposed in the IEEE 802.11ac standard.
In IEEE 802.11ac, a transmitter of a BSS (basis service set) of certain bandwidth is allowed to transmit radio signals, onto the shared wireless medium depending on clear channel assessment (CCA) sensing. For a BSS of certain bandwidth, a valid transmission sub-channel shall have bandwidth, allowable in the IEEE 802.11ac, equal to or smaller than the full bandwidth of the BSS and contains the designated primary sub-channel of the BSS. Based on the CCA sensing in the valid transmission bandwidths, the transmitter is allowed to transmit in any of the valid transmission sub-channels as long as the CCA indicates the sub-channel (or full channel) is idle. This dynamic transmission bandwidth scheme allows system bandwidth resource to be efficiently utilized.
An enhanced distributed channel access protocol (EDCA) is used in IEEE 802.11ac as a channel contention procedure for wireless devices to gain access to the shared wireless medium, e.g., to obtain a transmitting opportunity (TXOP) for transmitting radio signals onto the shared wireless medium. The simple CSMA/CA with random back-off contention scheme and low cost ad hoc deployment in unlicensed spectrum have contributed rapid adoption of WiFi systems. Typically, the EDCA TXOP is based solely on activity of the primary channel, while the transmit channel width determination is based on the secondary channel CCA during an interval (PIFS) immediately preceding the start of the TXOP. The basic assumption of EDCA is that a packet collision can occur if a device transmits signal under the channel busy condition when the received signal level is higher than CCA level.
In general, higher channel width transmission is both bandwidth and power efficient. First, in OFDM/OFDMA systems, higher number of subcarriers are achieved with reduced guard tones. Second, lower rate codes are more powerful than higher rate codes. Furthermore, higher channel width transmission causes less interference in dense deployment environment because the transmitting (TX) spectral density is lower. However, under the current EDCA channel contention procedure, the likelihood of higher channel width transmission is low.
This is because radio signal propagation range is determined by the TX spectral density and the channel propagation loss. Currently, the primary CCA levels are based on equal spectral density for all RX channel widths. For example, the CCA level/channel width (in unit of 20 MHz) is equal to −82 dBm for any 20 MHz, 40 MHz, 80 MHz, and 160 MHz RX channels. However, the TX spectral density is not the same for all TX channel widths. In general, the transmit power is nearly the same for all transmit channel widths, adjusted slightly based on peak average power ratio (PAPR) and/or EVM. The TX spectral density of a narrower TX channel therefore is higher than the TX spectral density of a wider TX channel, e.g., TX_PWR/20M>TX_PWR/40M>TX_PWR/80M>TX_PWR/160M. As a result, a narrower TX channel width transmission interferes (defers) a wider channel width transmission even though the wider channel width transmission might not cause interference to the narrower TX channel width transmission in the reciprocal direction. This is because the primary CCA level is set to fixed values regardless of what the transmit channel width is. Therefore, the likelihood of the wider channel transmission is reduced based on the current EDCA procedure.
Today, WiFi devices are over-populated. Dense deployment has led to significant issues such as interference, congestion, and low throughput. Raising CCA levels has been shown to increase spatial reuse, which leads to significant increase in the network throughput in dense deployment scenarios. However, raising CCA levels leads to high collision and starvation for legacy stations. A solution is sought to increase the likelihood of wider channel width transmission and to alleviate the above issues when raising the CCA levels and thereby increasing the network throughput.